Abstract

In quasi-gauge spaces (X,P) (in the sense of Dugundji and Reilly), we introduce the concept of the left (right) J-family of generalized quasi-pseudodistances, and we use this J-family to define the new kind of left (right) J-sequential completeness, which extends (among others) the usual P-sequential completeness. We use this J-family to construct more general contractions than those of Banach and Rus, and for such contractions (which are not necessarily continuous), we establish the conditions guaranteeing the existence of periodic points (when (X,P) is not Hausdorff), fixed points (when (X,P) is Hausdorff), and iterative approximation of these points. The results are new in quasi-gauge, topological and quasi-uniform spaces and, in particular, generalize the well-known theorems of Banach and Rus types in the matter of fixed points. Various examples illustrating ideas, methods of investigations, definitions and results, and fundamental differences between our results and the well-known ones are given.

MSC:54H25, 54A05, 47J25, 47H09, 54E15.

Keywords

1 Introduction

Let X be a nonempty set. If T:X→X, then, for each w0∈X, we define a sequence (wm:m∈{0}∪N) starting with w0 as follows ∀m∈{0}∪N{wm=T[m](w0)}, where T[m]=T∘T∘⋯∘T (m-times), and T[0]=IX is an identity map on X.

By Fix(T) and Per(T), we denote the sets of all fixed points and periodic points of T:X→X, respectively, i.e., Fix(T)={w∈X:w=T(w)} and Per(T)={w∈X:w=T[s](w) for some s∈N}.

Theorem 1.1If(X,d)is a complete metric space with metricd, then the mapT:X→Xsatisfying the condition

∃λ∈[0,1)∀x,y∈X{d(T(x),T(y))⩽λd(x,y)}

(1.1)

has a unique fixed pointwinX (i.e., Fix(T)={w}) and∀w0∈X{limm→∞wm=w}.

Another is a theorem of Rus [3] (see also [4, 5] and [6]), which states the following.

Theorem 1.2If(X,d)is a complete metric space with metricd, then a continuous mapT:X→Xsatisfying the condition

∃λ∈[0,1)∀x∈X{d(T(x),T[2](x))⩽λd(x,T(x))}

(1.2)

has the properties xFix(T)≠∅and∀w0∈X∃w∈Fix(T){limm→∞wm=w}.

It is clear that the map T satisfying (1.1) is continuous and satisfies (1.2), and in the assertion of Theorem 1.2, the uniqueness such as in the assertion of Theorem 1.1 does not necessarily hold.

These results are basic facts in the metric fixed point theory and their applications, and in the last four decades, the question concerning important generalizations of [1, 2] and [3] has received considerable attention from various researchers, and some very interesting results have been obtained in several hundred papers and several books. It is not our purpose to give a complete list of related papers and books here.

In important and various directions, there are elegant results discovered by [7–13], in which more general and natural settings, by using asymmetric structures in considerable spaces, are studied; in [7–9] a complete metric space (X,d) in results of [1–3] is replaced by a left (right) P-sequentially complete quasi-gauge space (X,P), and in construction of contractive conditions of (1.1) and (1.2) types, the quasi-gauge P is used, whereas [10] and [11–13] provide substantial and inspiring tools for investigations in complete metric spaces (X,d) the existence of fixed points of maps which are the contractions of [1–3] types with respect to w-distances and τ-distances, respectively.

Note that quasi-gauge P, w-distances and τ-distances generate asymmetric structures and generalize metric d, and that the studies of asymmetric structures and their applications in theoretical computer science are important.

Our main interest of this paper is the following.

Question 1.1 For which not necessarily Hausdorff and not necessarily complete spaces or not necessarily sequentially complete spaces and for which new families of distances on these spaces, there exist symmetric or asymmetric structures determined by these new families of distances which are more general than those determined by quasi-gauges P, w-distances, τ-distances or metrics d, and for which not necessarily continuous contractions of the Banach or Rus types with respect to these new families of distances the assertions such as in the results of [1, 2] or [3], respectively, hold (and not only for fixed points but also for periodic points)?

In this paper, in the quasi-gauge spaces (X,P) (see Definition 2.1), to answer this question affirmatively, we introduce the concepts of the left (right) J-families of generalized quasi-pseudodistances (see Definition 3.1), and we show how these left (right) J-families can be used, in a natural way, to define the left (right) J-sequential completeness (see Definition 3.2) which generalize (among others) the usual left (right) P-sequential completeness, to construct the not necessarily continuous contractions T:X→X of Banach and Rus types (see conditions (H1) and (H2)), and assuming additionally that T[s] is a left (right) P-quasi-closed map inX for some s∈N (see Definition 3.3), to obtain the new periodic and fixed point theorems (see Theorems 4.1 and 4.2) which, in particular, generalize Banach and Rus results in the matter of fixed points. The results are new in quasi-gauge, topological and quasi-uniform spaces (see Remarks 2.1, 3.1, 3.2 and 6.1). Various examples illustrating ideas, methods of investigations, definitions and results, and fundamental differences between our results and the well-known ones are given (see Section 6).

For given quasi-pseudometric p on X, a pair (X,p) is called quasi-pseudometric space. A quasi-pseudometric space (X,p) is called Hausdorff if ∀x,y∈X{x≠y⇒p(x,y)>0∨p(y,x)>0}.

(ii)

Each family P={pα:α∈A} of quasi-pseudometrics pα:X×X→[0,∞), α∈A, is called a quasi-gauge on X (A-index set).

(iii)

Let the family P={pα:α∈A} be a quasi-gauge on X. The topology T(P) having as a subbase the family

B(P)={B(x,εα):x∈X,εα>0,α∈A}

of all balls

B(x,εα)={y∈X:pα(x,y)<εα},x∈X,εα>0,α∈A,

is called the topology induced by P on X.

(iv)

(Dugundji [24], Reilly [7, 25]) A topological space (X,T) such that there is a quasi-gauge P on X with T=T(P) is called a quasi-gauge space and is denoted by (X,P).

(v)

A quasi-gauge space (X,P) is called Hausdorff if a quasi-gauge P has the property

∀x,y∈X{x≠y⇒∃α∈A{pα(x,y)>0∨pα(y,x)>0}}.

Remark 2.1 Each quasi-uniform space and each topological space is a quasi-gauge space (Reilly [[7], Theorems 4.2 and 2.6]).

3 Left (right) J-families, left (right) J-sequential completeness and left (right) P-quasi-closed maps in quasi-gauge spaces with generalized quasi-pseudodistances

We next record the definitions of left (right) J-families, left (right) J-sequential completeness and left (right) P-quasi-closed maps needed in the next sections.

Definition 3.1 Let (X,P) be a quasi-gauge space. The family J={Jα:α∈A} of maps Jα:X×X→[0,∞), α∈A, is said to be a left (right) J-family of generalized quasi-pseudodistances onX (left (right) J-family onX, for short) if the following two conditions hold:

(J1) ∀α∈A∀x,y,z∈X{Jα(x,z)⩽Jα(x,y)+Jα(y,z)}; and

(J2) for any sequences (um:m∈N) and (vm:m∈N) in X satisfying

∀α∈A∀ε>0∃k∈N∀n,m∈N;k⩽m<n{Jα(um,un)<ε}

(3.1)

(∀α∈A∀ε>0∃k∈N∀n,m∈N;k⩽m<n{Jα(un,um)<ε})

(3.2)

and

∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{Jα(vm,um)<ε}

(3.3)

(∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{Jα(um,vm)<ε}),

(3.4)

the following holds

∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{pα(vm,um)<ε}

(3.5)

(∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{pα(um,vm)<ε}).

(3.6)

Remark 3.1 If (X,P) is a quasi-gauge space, then P∈J(X,P)L, where

J(X,P)L={J:J is a left J-family on X}

and P∈J(X,P)R, where

J(X,P)R={J:J is a right J-family on X}.

One can prove the following proposition.

Proposition 3.1Let(X,P)be a Hausdorff quasi-gauge space, and letJ={Jα:X×X→[0,∞),α∈A}be a left (right) J-family onX. Then

∀x,y∈X{x≠y⇒∃α∈A{Jα(x,y)>0∨Jα(y,x)>0}}.

Proof Assume that J is a left J-family, and that there are x≠y, x,y∈X, such that ∀α∈A{Jα(x,y)=Jα(y,x)=0}. Then ∀α∈A{Jα(x,x)=0}, by using property (J1) in Definition 3.1, it follows that

∀α∈A{Jα(x,x)⩽Jα(x,y)+Jα(y,x)=0}.

Defining the sequences (um:m∈N) and (vm:m∈N) in X by um=x and vm=y or by um=y and vm=x for m∈N, observing that ∀α∈A{Jα(x,y)=Jα(y,x)=Jα(x,x)=0}, and using property (J2) of Definition 3.1 for these sequences, we see that (3.1) and (3.3) hold, and, therefore, (3.5) is satisfied, which gives ∀α∈A{pα(x,y)=pα(y,x)=0}. But this is a contradiction, since (X,P) is Hausdorff, and thus, x≠y⇒∃α∈A{pα(x,y)>0∨pα(y,x)>0}. When J is a right J-family, then the proof is based on the analogous technique. □

The necessity of defining the various concepts of completeness in quasi-gauge spaces became apparent with the investigation of asymmetric structures in these spaces. General results of this sort were progressively shown in a series of papers, and important ideas are to be found in [7–9, 24–27], which also contain many examples.

Now, using left (right) J-families, we define the following new natural concept of completeness.

Definition 3.2 Let (X,P) be a quasi-gauge space, and let J={Jα:X×X→[0,∞),α∈A} be a left (right) J-family on X.

(i)

We say that a sequence (um:m∈N) in X is left (right) J-Cauchy sequence inX if

We say that a sequence (um:m∈N) in X is left (right) J-convergent inX if

S(um:m∈N)L−J≠∅(S(um:m∈N)R−J≠∅),

where

S(um:m∈N)L−J={u∈X:limm→∞L−Jum=u}(S(um:m∈N)R−J={u∈X:limm→∞R−Jum=u}).

(iv)

If every left (right) J-Cauchy sequence (um:m∈N) in X is left (right) J-convergent in X

(i.e.,S(um:m∈N)L−J≠∅(S(um:m∈N)R−J≠∅)),

then (X,P) is called a left (right) J-sequentially complete quasi-gauge space.

Remark 3.2 (a) It is clear that if (wm:m∈N) is left (right) J-convergent in X, then

S(wm:m∈N)L−J⊂S(vm:m∈N)L−J(S(wm:m∈N)R−J⊂S(vm:m∈N)R−J)

for each subsequence (vm:m∈N) of (wm:m∈N) (see Example 3.1).

(b) There exist examples of quasi-gauge spaces (X,P) and left (right) J-family J on X, J≠P such that (X,P) is left (right) J-sequentially complete, but not left (right) P-sequentially complete (see Section 6).

Also, using Definition 3.2, we can define the following generalization of continuity.

Definition 3.3 Let (X,P) be a quasi-gauge space, let T:X→X, and let s∈N. The map T[s] is said to be a left (right) P-quasi-closed map if every sequence (wm:m∈N) in T[s](X), left (right) P-converging in X

(thus, S(wm:m∈N)L−P≠∅(S(wm:m∈N)R−P≠∅))

and having subsequences (vm:m∈N) and (um:m∈N) satisfying ∀m∈N{vm=T[s](um)} has the property

(iii) Since in the results of [1, 2] and [4], the spaces (X,d) are Hausdorff and complete, and the maps T:X→X are continuous, therefore, Theorems 4.1 and 4.2 are new generalizations of [1, 2] and [3], respectively; more precisely, the assertions are identical, but assumptions are weaker.

(iv) The statements (C) and (F) say that each periodic point is a fixed point when (X,P) is Hausdorff; for illustrations, see Examples 6.1-6.7.

(v) The situations when (X,P) is not Hausdorff and the periodic points exist but they are not fixed points are described in Examples 6.8 and 6.9.

5 Proofs

We prove Theorems 4.1 and 4.2 in the case when J is left J-family and a quasi-gauge space (X,P) is left J-sequentially complete; we omit the proof when J is a right J-family and (X,P) is right J-sequentially complete, which is based on the analogous technique.

Proof of Theorem 4.2 (D) The assertion (d1) holds. The proof will be broken into four steps.

Step D.I. The following holds:

∀α∈A∀w0∈X{limm→∞sup{Jα(wm,wn):n>m}=0}.

Indeed, if α∈A and w0∈X are arbitrary and fixed, m,n∈N and n>m, then by (J1) and (H2), we get that

Indeed, let w0∈X be arbitrary and fixed. By (5.1) and Definition 3.2(i), the sequence (wm:m∈{0}∪N) is left J-Cauchy on X. Hence, since (X,P) is a left J-sequentially complete quasi-gauge space, we get that (wm:m∈{0}∪N) is left J-convergent in X, i.e., there exists, by Definition 3.2(ii)-(iv), a nonempty set S(wm:m∈{0}∪N)L−J⊂X, such that for all w∈S(wm:m∈{0}∪N)L−J, we have

∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{Jα(w,wm)<ε}.

(5.2)

However, J is left J-family. Therefore, from (5.1) and (5.2), fixing w∈S(wm:m∈{0}∪N)L−J, defining (um=wm:m∈{0}∪N) and (vm=w:m∈{0}∪N) and using Definition 3.1 for these sequences, we conclude that

∀α∈A∀ε>0∃k∈N∀m∈N;k⩽m{pα(w,wm)<ε},

i.e., limm→∞L−Pwm=w. Clearly, this means that S(wm:m∈{0}∪N)L−P≠∅.

We proved that the assertion (d1) holds.

(E)

The assertions of (e1)-(e3) hold.

The proof will be broken into three steps.

Step E.I. We show that (e1) holds. Indeed, let w0∈X be arbitrary and fixed. By (D), S(wm:m∈{0}∪N)L−P≠∅, and since

w(m+1)s=T[s](wms)for m∈{0}∪N,

thus, defining (wm=wm−1+s:m∈N), we see that

(wm:m∈N)⊂T[s](X),S(wm:m∈{0}∪N)L−P=S(wm:m∈{0}∪N)L−P≠∅,

the sequences

(vm=w(m+1)s:m∈N)⊂T[s](X)

and

(um=wms:m∈N)⊂T[s](X)

satisfy

∀m∈N{vm=T[s](um)}

and, as subsequences of (wm:m∈{0}∪N), are left P-converges to each point of w∈S(wm:m∈{0}∪N)L−P. Moreover, by Remark 3.2(a),

S(wm:m∈N)L−P⊂S(vm:m∈N)L−PandS(wm:m∈N)L−P⊂S(um:m∈N)L−P.

By above, since T[s] is left P-quasi-closed for some s∈N, we conclude that

∃w∈S(wm:m∈{0}∪N)L−P=S(wm:m∈N)L−P{w=T[s](w)}.

Consequently, (e1) holds.

Step E.II. We show that (e2) holds. Assertion (e2) follows from assertion (d1) and Step E.I.

Step E.III. We show that (e3) holds. Assume that w∈Fix(T[s]) is arbitrary and fixed.

(II.1) We show that(X,P)is not a leftP-sequentially complete quasi-gauge space.

Indeed, let (um=1/2m:m∈N). By (6.1),

∀ε>0∃k0∈N∀n,m∈N;k0⩽m⩽n{p(um,un)=|1/2m−1/2n|<ε}.

Thus, this sequence is left P-Cauchy. However, this sequence in not left P-convergent in X. Otherwise, supposing that limm→∞L−Pum=u for some u∈X we may assume, not losing generality, that

∀0<ε<1∃k0∈N∀m∈N;k0⩽m{p(u,um)<ε<1}.

(6.2)

Then, the following two cases hold:

Case 1. If u∉A, then, by (6.1), since ∀m∈N{um∈A}, we have

∀m∈N;k0⩽m{p(u,um)=|u−um|+1<ε<1},

which is impossible;

Case 2. If u∈A, then u=1/2k1 for some k1∈N and, using (6.1), we see that

∀m∈N;k0⩽m{p(u,um)=|u−um|=|1/2k1−1/2m|},

and taking the limit interior as m→∞, we find limm→∞p(u,um)=1/2k1, which, by (6.2), is impossible.

We conclude that (X,P) is not a left P-sequentially complete.

Example 6.3 Let (X,P) be a quasi-pseudometric space, where P={p} and p is a quasi-pseudometric on X. Let the set E⊂X, containing at least two different points, be arbitrary and fixed, and let c>0 satisfy δ(E)<c, where

δ(E)=sup{p(x,y):x,y∈E}.

Define J:X×X→[0,∞) by

J(x,y)={p(x,y)if E∩{x,y}={x,y},cif E∩{x,y}≠{x,y},x,y∈X.

(6.3)

(III.1) The familyJ={J}is leftJ-family onX.

Indeed, it is worth noticing that condition (J1) does not hold only if there exist some x0,y0,z0∈X satisfying

J(x0,z0)>J(x0,y0)+J(y0,z0).

This inequality is equivalent to

c>p(x0,y0)+p(y0,z0),

where J(x0,z0)=c, J(x0,y0)=p(x0,y0) and J(y0,z0)=p(y0,z0). However, by (6.3), we get the following.

We omit the proof since it is based on the analogous technique as in (III.1).

Example 6.4 Let X=[0,1]⊂R, P={p} and p:X×X→[0,∞), where p is such as in Example 6.1. Let E=[1/8,1], and let J:X×X→[0,∞) be given by the formula

J(x,y)={p(x,y)if {x,y}∩E={x,y},4if {x,y}∩E≠{x,y}.

(6.6)

(IV.1) J={J}is a leftJ-family onX.

This follows from (III.1).

(IV.2) (X,P)is not a leftP-sequentially complete quasi-gauge space.

This follows from (II.1).

(IV.3) (X,P)is a leftJ-sequentially complete quasi-gauge space.

Indeed, let (um:m∈N) be a left J-Cauchy sequence; not losing generality, we may assume that

∀0<ε<1/8∃k0∈N∀n,m∈N;k0⩽m⩽n{J(um,un)<ε<1/8}.

(6.7)

Then, by (6.7), (6.6) and (6.1), we get

∀0<ε<1/8∃k0∈N∀n,m∈N;k0⩽m<n{J(um,un)=p(um,un)=|um−un|<ε<1/8},

(6.8)

∀m∈N;k0⩽m{um∈E=[1/8,1]}

(6.9)

and

∀m∈N;k0⩽m{um∈A or um∉A}.

(6.10)

We consider the following two cases.

Case 1. Let ∀l∈N{uk0+l∈A}. This together with (6.8)-(6.10) shows that ∀l∈N{uk0+l=1/2} or ∀l∈N{uk0+l=1/4} or ∀l∈N{uk0+l=1/8} and, therefore, the sequence (um:m∈N) is left J-convergent to the point 1/2 or 1/4 or 1/8, respectively;

Case 2. Let ∃l0∈N{uk0+l0∉A}. We note that then

∀n>l0{uk0+n∉A}.

(6.11)

Otherwise, S={n>l0:uk0+n∈A}≠∅, and let s0=minS. By definition of S, uk0+s0−1∉A and uk0+s0∈A, which, by (6.6) and (6.1), gives

J(uk0+s0−1,uk0+s0)=p(uk0+s0−1,uk0+s0)=|uk0+s0−1−uk0+s0|+1,

and this, by (6.7), is impossible. Thus, (6.11) holds. Now, since (R,|⋅|) is complete, E=[1/8,1] is closed in ℝ, ∀m⩾k0{um∈E} by (6.9), and (um:m∈N) is Cauchy with respect to |⋅| (indeed, by (6.8), we get that

∀0<ε<1/8∃k0∈N∀n,m∈N;k0⩽m⩽n{|um−un|<ε}

holds), thus, there exists u∈E such that

∀0<ε<1/8∃k1∈N∀m∈N;k1⩽m{|u−um|<ε}.

(6.12)

Next, by (6.11) and (6.12),

∀0<ε<1/8∃m0∈N,m0=max{k1,k0+l0}∀m∈N;m0⩽m{um∉A∧|u−um|<ε},

which, by (6.6) and (6.1), implies that

∀0<ε<1/8∃m0∈N∀m∈N;m0⩽m{J(u,um)=p(u,um)=|u−um|<ε},

and we conclude that (um:m∈N) is left J-convergent to u.

This means that (X,P) is left J-sequentially complete.

Theorem 4.2 is quite general, and does not require left P-sequential completeness; in Example 6.5, T satisfies (H2) for some J≠P, and λ=3/4, T satisfies (H2) for J=P and λ=1/2, and (X,P) is left J-sequentially complete but not left P-sequentially complete.

Example 6.5 Let X, P={p}, E and J be as in Example 6.4, and let T:X→X be given by

Case 3. If x∈[1/8,1/2)∩A={1/8,1/4}⊂E, then, by (6.13) and (6.14), T(1/8)=5/16, T[2](1/8)=13/32, T(1/4)=3/8 and T[2](1/4)=7/16. Therefore, {T(x),T[2](x)}∩A=∅ and {T(x),T[2](x)}⊂E. Hence, by (6.6) and (6.1), we get

Case 5. If x∈[1/2,1] then {T(x),T[2](x)}={1/2}⊂A. Moreover, {T(x),T[2](x)}⊂E. Hence, by (6.6) and (6.1),

J(T(x),T[2](x))=|T(x)−T[2](x)|=|1/2−1/2|=0⩽λJ(x,T(x)).

Consequently, the map T satisfies (H2) for λ=3/4 and J defined by (6.6).

(V.3) Tis leftP-quasi-closed onX.

Indeed, let (wm:m∈N) be arbitrary and fixed sequence in T(X)=[1/4,1/2], left P-convergent to each point of a nonempty set S(wm:m∈N)L−P, and having subsequences (vm:m∈N) and (um:m∈N) satisfying ∀m∈N{vm=T(um)}.

Let w∈S(wm:m∈N)L−P be arbitrary and fixed. Then, by (6.1), (6.13) and Definition 3.2, we conclude that

Now, we notice that the existence of J-family such that J≠P is essential; in Example 6.6, T satisfies (H2) for some J≠P and does not satisfy (H2) for J=P, and (X,P) is left J-sequentially complete but not left P-sequentially complete.

Example 6.6 Let X, P={p}, E and J={J} be as in Example 6.4. Define T:X→X by

T(x)={1if x∈[0,1/8),x/2+1/4if x∈[1/8,1/2),1/2if x∈[1/2,1].

(6.15)

(VI.1) ForJ={J}, (X,P)isJ-sequentially complete.

This follows from (IV.3).

(VI.2) Tsatisfies (H2) forλ=3/4and forJdefined in (6.6).

Indeed, we get

T[2](x)={1/2if x∈[0,1/8)∪[1/2,1],x/4+3/8if x∈[1/8,1/2),

(6.16)

and using (6.15) and (6.16), we consider the following four cases.

Case 1. If x∈[0,1/8), then x∉E and, by (6.15) and (6.16), T(x)=1∉A, T(x)∈E, T[2](x)=1/2∈A∩E. Consequently, by (6.6) and (6.1),

Case 4. If x∈[1/2,1], then T(x)=T[2](x)=1/2∈A∩E. Hence, by (6.6) and (6.1),

J(T(x),T[2](x))=|T(x)−T[2](x)|=0⩽λJ(x,T(x)).

Consequently, for λ=3/4 and J defined in (6.6) and (6.1), the map T satisfies condition (H2).

(VI.3) Tis leftP-quasi-closed onX.

Indeed, let (wm:m∈N) be arbitrary and fixed sequence in T(X)=[5/16,1/2]∪{1}, left P-convergent to each point of a nonempty set S(wm:m∈N)L−P⊂X and having subsequences (vm:m∈N) and (um:m∈N) satisfying ∀m∈N{vm=T(um)}.

Let w∈S(wm:m∈N)L−P be arbitrary and fixed. Then, by (6.1), (6.13) and Definition 3.2, we conclude that

Case 1. If x∈{0} then, by (6.18) and (6.20), T(x)=T[2](x)=0∈E, so, by (6.19) and (6.17),

J(T(x),T[2](x))=0⩽λJ(x,T(x));

Case 2. If x∈[3,5]∪{6}, then, by (6.18) and (6.20), T(x)=T[2](x)=3∈E, so by (6.19) and (6.17),

J(T(x),T[2](x))=0⩽λJ(x,T(x));

Case 3. If x∈(0,1)∪(2,3), then x∈E and, by (6.18) and (6.20), T(x)=6∈E, T[2](x)=3∈E, so by (6.19) and (6.17),

J(T(x),T[2](x))=p(6,3)=0⩽λJ(x,T(x));

Case 4. If x∈(5,6), then, by (6.18) and (6.20), {T(x),T[2](x)}={0}⊂E, so by (6.19) and (6.17), J(T(x),T[2](x))=p(0,0)=0. Hence,

J(T(x),T[2](x))=0⩽λJ(x,T(x));

Case 5. If x=1, then, by (6.18) and (6.20), T(x)=6⊂E, T[2](x)=3⊂E. Since T(x)>T[2](x), by (6.19) and (6.17), J(T(x),T[2](x))=J(6,3)=0. Therefore,

J(T(x),T[2](x))=0⩽λJ(x,T(x));

Case 6. If x=2, then, by (6.18) and (6.20), T(x)=5/2∈E and T[2](x)=6∈E. Since T(x)<T[2](x), by (6.19) and (6.17), J(T(x),T[2](x))=1. But x∉E and, by (6.19) and (6.17), J(x,T(x))=4. Therefore,

J(T(x),T[2](x))=1⩽4/3=(1/3)4=λJ(x,T(x));

Case 7. If x∈(1,2), then, by (6.18) and (6.20), T(x)=x/2+3/2∈(2,5/2)⊂E and T[2](x)=6∈E. Since T(x)<T[2](x), by (6.19) and (6.17), J(T(x),T[2](x))=1. But x∉E and, by (6.19) and (6.17), J(x,T(x))=4. Therefore,

J(T(x),T[2](x))=1⩽4/3=(1/3)4=λJ(x,T(x)).

Consequently, for λ=1/3 and J defined in (6.19) and (6.17), the map T satisfies condition (H2).

(VII.6) Condition (E1) holds.

Indeed, we prove that T[3] is left P-quasi-closed on X. With this aim, we see that, by (6.18) and (6.20),

T[3](x)={3if x∈(0,5]∪{6},0if x∈{0}∪(5,6),

(6.21)

and let (wm:m∈N) be an arbitrary and fixed sequence in T[3](X)={0,3}, left P-convergent to each point of a nonempty set S(wm:m∈N)L−P⊂X, and having subsequences (vm:m∈N)⊂T[3](X) and (um:m∈N)⊂T[3](X) satisfying ∀m∈N{vm=T[3](um)}. Clearly, S(wm:m∈N)L−P⊂S(vm:m∈N)L−P and S(wm:m∈N)L−P⊂S(um:m∈N)L−P. Hence, by (6.21), we obtain (vm:m∈N)⊂{0,3} and (um:m∈N)⊂{0,3}, which gives the following.

Case 1. If (wm:m∈N) and (vm:m∈N) are such that ∃m0∈N∀m⩾m0{vm=0}, then also ∀m⩾m0{um=0}. Consequently,

[0,6]=S(vm:m∈N)L−P=S(um:m∈N)L−P;

Case 2. If (wm:m∈N) and (vm:m∈N) are such that ∃m0∈N∀m⩾m0{vm=3} or ∀m0∈N∃m1⩾m0∃m2⩾m0{vm1=0∧vm2=3}, then, by (6.21), also ∀m⩾m0{um=3} or {um1=0∧um2=3}. Consequently,

[3,6]=S(vm:m∈N)L−P=S(um:m∈N)L−P.

Of course, since (wm:m∈N)⊂T[3](X)={0,3}, therefore, [3,6]⊂S(wm:m∈N)L−P. Finally, we see that ∃w=3∈S(wm:m∈N)L−P{w=T[3](w)} in Cases 1 and 2. By Definition 3.3, T[3] is left P-quasi-closed on X.

Indeed, from (6.22), we have that p(x,x)=0 for each x∈X, and thus, condition (P1) holds.

Now, it is worth noticing that condition (P2) does not hold only if there exists x0,y0,z0∈X such that p(x0,z0)>p(x0,y0)+p(y0,z0). This inequality is equivalent to 1>0=p(x0,y0)+p(y0,z0), where

p(x0,z0)=1,

(6.23)

p(x0,y0)=0

(6.24)

and

p(y0,z0)=0.

(6.25)

Conditions (6.24) and (6.25) imply that x0=y0 or {x0,y0}⊂A and y0=z0 or {y0,z0}⊂A, respectively. We consider the following four cases.

Case 1. If x0=y0 and y0=z0, then x0=z0 which, by (6.22), implies that p(x0,z0)=0. By (6.23), this is absurd;

Case 2. If x0=y0 and {y0,z0}⊂A, then {x0,z0}∩A={x0,z0}. Hence, by (6.22), p(x0,z0)=0. By (6.23), this is absurd;

Case 3. If {x0,y0}⊂A and y0=z0, then {x0,z0}∩A={x0,z0}. Hence, by (6.22), p(x0,z0)=0. By (6.23), this is absurd;

Case 4. If {x0,y0}⊂A and {y0,z0}⊂A, then {x0,z0}∩A={x0,z0}. Hence, by (6.22), p(x0,z0)=0. By (6.23), this is absurd.

Thus, condition (P2) holds.

We proved that p is quasi-pseudometric on X, and (X,P) is the quasi-gauge space.

(VIII.2) The quasi-gauge space(X,P)is not Hausdorff.

Indeed, for x=1/16 and y=1/4 we have x≠y and {x,y}∩A={x,y}. Hence, by (6.22), we obtain p(x,y)=p(y,x)=0. This, by Definition 2.1(v), means that (X,P) is not Hausdorff.

Example 6.9 Let X=[0,1]⊂R, let P={p}, where p is defined as in Example 6.8, and let T:X→X be given by the formula

T(x)={1/2if x∈[0,1/4],1/4if x∈(1/4,1].

(6.26)

(IX.1) The pair(X,P)is a not a Hausdorff quasi-gauge space.

This is a consequence of (VIII.1) and (VIII.2).

(IX.2) The space(X,P)is a leftP-sequentially complete.

Indeed, let (um:m∈N) be a left P-Cauchy sequence in X. By (6.22), not losing generality, we may assume that

∀0<ε<1∃k0∈N∀m,n∈N;k0<m<n{p(um,un)=0<ε<1}.

(6.27)

Now, we have the following two cases.

Case 1. Let ∀m∈N;k0<m{um∈A}. By (6.22), in particular, we have that ∀m>k0{p(1/2,um)=0}. This gives, 1/2∈S(um:m∈{0}∪N)L−P, i.e., S(um:m∈{0}∪N)L−P≠∅;

Case 2. Let ∃m0∈N;k0<m0{um0∉A}. Then we have the following two subcases: Subcase 2.1. If ∀m∈N;k0<m,m≠m0{um=um0}, then, by (6.22), we get ∀m∈N;m0<m{p(um0,um)=0}, and this implies that um0∈S(um:m∈{0}∪N)L−P, i.e., S(um:m∈{0}∪N)L−P≠∅; Subcase 2.2. If ∃m1∈N;k0<m1,m1≠m0{um1≠um0}, then, by (6.22), p(um1,um0)=1. However, since k0<m0 and k0<m1, this, by (6.27), implies that p(um1,um0)=0. This is absurd.

We proved that if (6.27) holds, then

S(um:m∈{0}∪N)L−P≠∅.

By Definition 3.2(ii), the sequence (um:m∈N) is left P-convergent in X.

and let (wm:m∈N) be an arbitrary and fixed sequence in T[2](X)={1/4,1/2}, left P-convergent to each point of a nonempty set S(wm:m∈N)L−P⊂X and having subsequences (vm:m∈N) and (um:m∈N) satisfying ∀m∈N{vm=T[2](um)}. Thus, (wm:m∈N)⊂{1/4,1/2}⊂A, (vm:m∈N)⊂{1/4,1/2}⊂A and (vm:m∈N)⊂{1/4,1/2}⊂A. Hence, by (6.22), we conclude that